LORD OWEN yesterday urged 'relentless pressure' on the Bosnian Serbs, as the European Community threatened unilateral sanctions against Belgrade.

The campaign to force acceptance of the peace plan brokered by Lord Owen and Cyrus Vance has hit top gear, with a draft resolution circulating at the UN Security Council that would tighten the economic noose on Serbia and Montenegro. But there are disturbing signs that the coalition of the EC, the United States and Russia is breaking up, and Lord Owen is trying to pull things together.

After addressing a meeting of EC foreign ministers in Luxembourg yesterday, he said: 'Everyone believes this is the time to put relentless pressure on the Bosnian Serbs and the governments of Serbia and Montenegro to accept the peace plan which has been put before them.'

The question for the Bosnian Serbs, Lord Owen said, was: 'Are they going to drag the whole of Serbia and Montenegro down into isolation with them?' He added: 'If Belgrade decides that it wants this done, then it will happen.' The first step is to tighten existing sanctions against the rump Yugoslavia. The Western European Union, the defence body with ties to the EC, yesterday agreed to send patrol vessels and customs teams to support sanctions operations to the Danube states.

And the diplomatic machine is preparing new moves. The EC came out with a toughly worded statement, saying that 'non-acceptance of the peace plan would have the most severe consequences and would lead to total international isolation of Serbia-Montenegro'.

A resolution to toughen the sanctions has been circulating at the UN for five days. It should be voted on in the next two days. It would ban all transit traffic on the Danube apart from humanitarian shipments agreed in advance with New York.

But the new UN resolution does not go nearly as far as the EC would have liked in tightening the noose around Serbia. Diplomats said yesterday that Russia had forced the removal of some of the tougher elements in the sanctions package, including total diplomatic isolation and cutting telephone links.

The EC said that it might go ahead with its own action if the UN was not tough enough. 'We should apply maximum pressure on the Serbs and I would not exclude unilateral action,' said Neils Helveg Petersen, the Danish Foreign Minister and President of the EC Council of Ministers.

But Britain is intent on sticking with the UN line, said Tristan Garel Jones, Minister for Europe at the Foreign Office. He said that '95 per cent' of what the EC wants is in the UN draft resolution, and insisted that all the action should take place in the Security Council.